In a high intensity discharge (HID) lamp, a medium-to-high pressure ionizable gas, such as mercury or sodium vapor, emits visible radiation upon excitation typically caused by passage of current through the gas. In the original class of HID lamps, discharge current was caused to flow between two electrodes. However, a major cause of early electroded HID lamp failure has been found attributable to at least two inherent operational characteristics of such lamps. First, during lamp operation, sputtering of electrode material onto the lamp envelope is common and impedes optical output. Second, thermal and electrical stresses often result in electrode failure.
Electrodeless HID lamps do not exhibit these life shortening phenomena found in electroded HID lamps. One class of electrodeless HID lamps involves generating an arc discharge by establishing a solenoidal electric field in the gas; and, hence, these lamps are referred to as HID-SEF lamps. In particular, a solenoidal electric field is created by the varying magnetic field of an excitation coil. Disadvantageously, it is difficult to develop a sufficiently high electric field gradient especially in the associated excitation coil, because the coil current may need to be prohibitively high, even if the current is provided in the form of pulses. Furthermore, providing a sufficiently high electric field gradient may be impossible, because the necessary voltage-per-turn of a particular excitation coil may exceed the turn-to-turn electrical breakdown voltage thereof.
Capacitive starting electrodes for HID-SEF lamps are described in commonly assigned, copending U.S. patent application of H.L. Witting, Ser. No. 225,315, filed on July 28, 1988, which is hereby incorporated by reference. A pair of starting electrodes, each comprising a conductive ring, is adjacent to the lamp envelope surrounding the arc tube and connected to the excitation coil. Coupling a high voltage signal between the pair of starting electrodes causes an electric field to be produced therebetween, which is of sufficient magnitude to create a glow discharge in the arc tube due to the arc tube wall capacitance. Heat sensitive members, e.g. bimetallic strips, are utilized for moving the starting electrodes away from the arc tube after initiating a plasma discharge, thereby extending the useful life of the lamp.
A spiral starting electrode for an HID-SEF lamp is described in commonly assigned, copending U.S. patent application of H.L. Witting, Ser. No. 226,584, filed on Aug. 1, 1988, which is hereby incorporated by reference. A single, conical-spiral-shaped starting electrode is positioned so that its narrower end is adjacent to, or on, the arc tube surface. The wider end of the starting electrode is positioned so that flux generated by the excitation coil cuts the turns of the spiral electrode, thereby generating a high-voltage signal which results in a sufficiently high electric field gradient to create a glow discharge in the arc tube. Preferably, a bimetallic strip is utilized to move the starting electrode away from the arc tube after a plasma discharge is initiated therein.
A pair of starting electrodes which are moved from a rest position to a starting location adjacent to the arc tube by piezoelectric means is described in commonly assigned, copending U.S. patent application of J.C. Borowiec, Ser. No. 229,187, filed on Aug. 8, 1988, which is hereby incorporated by reference. The piezoelectric means are inactivated after a plasma discharge is initiated, and the starting electrodes are moved back to the rest position. Advantageously, the piezoelectric means allows the selective movement of the starting electrodes, thereby enabling the lamp to be restarted, if necessary, even if the arc tube is still hot.
Another approach to initiating a plasma discharge within the arc tube of an HID-SEF lamp is described in commonly assigned, copending U.S. patent application of H.L. Witting, Ser. No. 240,331, filed on Sept. 6, 1988, which is hereby incorporated by reference. In particular, an excitation coil has first and second substantially cone-shaped, solenoidally wound coil portions. The narrow end of each coil portion is adjacent to the arc tube. At any instant, the magnetic fields produced by each of the two coil portions combine in an in-phase manner to produce a sufficiently high electric field across the arc tube to initiate and maintain a plasma discharge therein.